1. Field of the Invention
The invention relates to pulse modulated RF signal sources and more particularly to a pulse modulated RF signal source useful as an altitude simulator to test radar altimeters.
2. Description of the Prior Art
A functional block diagram of a typical prior art altitude simulator for testing radar altimeters is illustrated in FIG. 1.
A typical radar altimeter comprises two basic parts referred to as the transmitter 10 and the receiver 11. The primary purpose of the transmitter 10 is to send out pulses of RF energy via the antenna (not illustrated) which are bounced off the surface of the earth and returns to the receiver 11. The receiver 11 converts the returned RF signal to a pulsed signal and measures the time delay between each of the transmitted pulses and the associated return signals arriving at the receiver 11 and calculates the altitude of the radar altimeter with respect to the earth.
To facilitate testing of the system, a typical transmitter 10 will provide a pulsed trigger signal synchronized in frequency and phase coherent with the transmitted pulses of RF energy. To test the radar altimeter, this pulsed trigger signal is coupled to a range simulator 12 via a cable 13. The range simulator 12 includes a plurality of delay lines, labeled #1 through #n, with typical delay lines functionally illustrated at reference numerals 15, 16 and 17. The pulsed trigger signal from the transmitter 10 is coupled to the input of one of these delay lines by closing one of three switches 20, 21 and 22. Assuming that the switch illustrated at reference numeral 20 is the closed position and the other two switches 21 and 22 are open, the pulsed trigger signal from the transmitter 10 will be coupled to the input of the first delay line 15. The output terminals of delay lines #1 through #n are selectively coupled to the input of a return cable 25 by closing either one of three output switches 26, 27 and 28. Since for purposes of illustration, the first switch illustrated at reference numeral 20 is assumed to be closed coupling the trigger signal to the input of the first delay line 15, it is necessary to close the output switch 26 connecting the output terminal of the first delay line 15 to the return cable 25. Closing the switch 26 couples the output of the first delay line 15 to the input of the return cable 25 to produce at the input to the receiver 11 a pulsed signal which is delayed from the trigger signal from the transmitter 10 by the transmission delay of the first delay line 15 and its associated cables and switch circuits. The receiver utilizes this signal instead of the pulsed signal generated from the returned RF signals. If the first delay line 15 has been carefully measured, this delay can be correlated with a particular altitude to check the performance of the radar altimeter calibration at one specific altitude. Similarly, delay lines #2 through #n can be used to check the radar altimeter calibration at other selected altitudes.
The prior art simulator has several functional limitations. First, each of the delay lines must be carefully cut to simulate accurately one altitude. Thus, if it is desired to change the test altitude, the simulator must be physically modified. Perhaps even more serious is the limitation that for low altitude simulation, a large portion of the delay is in the cables and switching portions of the simulator. This can make calibration of the simulator, especially for low altitude altimeters extremely difficult.